An N.times.N' star coupler is one of the key elements in Local Area Network (LAN) applications of optical fibre. The simplest single-mode 2.times.2 star coupler can be manufactured by bringing the cores of two single-mode fibres sufficiently close together over an appropriate coupling length. Various such structures have been built by using etching, grinding and polishing, or fusion. A 2.times.2 star coupler can be used as a basic building block to construct larger N.times.N' couplers where N is equal to an arbitrary power of two. However, this involves interconnecting a large number of 2.times.2 couplers, increasing the excess loss for larger values of N.
European patent application number 0340987, published Nov. 8, 1989, [which is incorporated herein by reference] discloses an N.times.N' star optical coupler comprising a dielectric slab and two arrays of strip waveguide formed on a glass substrate. Opposite surfaces of the dielectric slab, to which the strip waveguides are attached, are curved. The radius of curvature and the distance between the surfaces are such that the optical axis of each waveguide at one surface extends radially across the slab to the centre of the other curved surface.
The configuration is said to provide even distribution of light from each waveguide to the waveguide at the opposite side of the dielectric slab. The optimized efficiency of such a coupler varies between 0.34 at the edge and 0.55 at the middle of the array, which is not entirely satisfactory. This gives better coupling efficiency compared with a slab having parallel sides, in which light from a particular input waveguide will cover more than the entire area of the opposite face, so it is relatively inefficient since much of the light is diffused before it reaches the output side of the coupler.
U.S. Pat. No. 4,057,319 discloses a coupler connecting one fibre in a bundle to the fibre in another bundle. A phase hologram plate is interposed between an input bundle of fibres and the output bundle of fibres. The phase hologram effectively focuses the light onto the output optical fibre and so improves coupling efficiency. A disadvantage of this device is that it is suitable only for individual connections and hence not suitable for applications requiring N.times.N' coupling.
U.S. Pat. No. 4,838,630, issued Jun. 13, 1989 [and incorporated herein by reference] discloses a planar optical interconnector for 1.times.N or N.times.1 coupling in interconnecting integrated circuits. The interconnector comprises a Bragg planar volume hologram which distributes optical signals, but is not capable of N.times.N' coupling.
U.S. Pat. No. 4,705,344, issued Nov. 10, 1987, [which is incorporated herein by reference] disclosed an interconnection device for optically interconnecting a plurality of optical devices. The interconnection device comprises an optically transparent spacer with photosensitive material on its opposite sides. Fringes are formed, fixedly positioned, on one of the surfaces. The fringes comprise a plurality of "sub-holograms". The other surface has positions for the optical devices. The fringe pattern is formed by directing a coherent light beam through the spacer and photosensitive material to one position and directing a second coherent light beam from a second position to interfere with the first beam. Each source device emits a light beam which traverses the transparent spacer, is reflected by the holograph on the opposite face, and returns to a different position. The hologram is, in effect a plurality of discrete holograms each one dedicated to one pair of positions. This kind of interconnection device provides logic functions for optical computing but is limited to 1.times.N coupling.
Thus, none of these known devices can provide N by N coupling with an efficiency and simplicity which can be considered satisfactory.
There remains a need for an optical interconnector with improved coupling efficiency for use in coupling single mode waveguides, for example optical fibres, in a number of applications such as local area networks, back planes of telephone switches and also in integrated circuits or circuit boards and similar situation where a large number of connections need to be made in a very limited space.